The present invention relates to an improvements in a light beam scanning system utilizing a rotary multi-surfaced mirror.
Recently, a number of devices have been developed for reading and recording information using a scanning light beam such as laser beams. A rotary multi-surfaced or faceted mirror is typically employed as a light deflector in such devices. However, deviations from a perfect parallel state often exist between the rotary shaft and the reflecting surfaces of such a mirror even if the components are manufactured with high precision. Such errors cause the scanning beam to be undersirably displaced perpendicular to its deflection direction. That is, the loci of the scanned light beams on the image plane due to reflection by the various surfaces of the rotary multi-surfaced mirror form not a straight, narrow line but instead a line with a significant deviation therefrom.
Methods for forming coincident scan lines by optically correcting lateral displacement caused by parallelism imperfections have been proposed. As an example Japanese Patent Publication No. 52-28666 discloses a method such as illustrated here in FIGS. 1(A) and 1(B) which are exploded views illustrating the method of this publication. FIGS. 1(A) and 1(B) show, respectively, states of a light beam via a rotary multisurfaced mirror in a scanning direction of the light beam and in a direction perpendicular to the scanning direction. According to this method, the light beam is converged only in a direction perpendicular to its scanning direction by a rotary multisurfaced mirror by using a cylindrical lens 11 to thereby form a linear image in the scanning direction on a reflecting surface 15. The light beam reflected by the reflecting surface is collimated by collimated by a second cylindrical lens 12 and finally a point image is formed on a scanning surface 16 by a convergent lens 13. In this case, as viewed in a direction perpendicular to the scanning direction of the light beam, a conjugate relationship between the object and image points is established between the linear image on the reflecting surface 15 and the point image focussed on the scanning surface 16. Even if the direction of the light beam reflected by the reflecting surface 15 is changed due to the parallelism deviations, the light beam is converged at a constant position on the scanning surface 16. Thus, the loci of the scan lines are made coincident with each other.
Since the second cylindrical lens 12 is disposed between the reflecting surface 15 and the convergent lens 13, the following relationship must be established: EQU f.sub.12 &lt;f.sub.13. (1a)
Furthermore, in order to form a locus of the point image focussed on the scanning surface as a circle the following relationship must be established: EQU f.sub.11 =f.sub.12, (1b)
where f.sub.11 and f.sub.12 are the focal lengths of the first and second cylindrical lenses 11 and 12, respectively, and f.sub.13 is the focal length of the convergent lens 13.
Both focal lengths f.sub.11 and f.sub.12 of the first and second cylindrical lenses must be shorter than the focal length f.sub.13 of the convergent lens. For example, when f.sub.13 =50 mm, f.sub.11 and f.sub.12 become about 30 to 35 mm. It is very difficult to manufacture such short focal length cylindrical lenses of a high quality. Also, if the focal length f.sub.11 of the first cylindrical lens is shortened to this extent, since the depth of focus thereof is narrow, the convergence of the light beam on the scanning surface 16 is varied due to the displacement of the reflecting surface 15.
Laid Open Japanese Utility Model Application No. 53-91845 discloses a method for eliminating the above-described disadvantage. This method is illustrated by an exploded view shown in FIGS. 2(A) and 2(B). FIGS. 2(A) and 2(B) show states of the light beam in the scanning direction of the rotary multi-surfaced mirror and in a direction perpendicular to the scanning direction. According to this method, the light beam is converged by a first cylindrical lens 21 only in a direction perpendicular to the scanning direction of the light beam from a rotary multi-surfaced mirror. A linear image is formed on a reflecting surface 25 of the rotary multi-surfaced mirror in parallel with the scanning direction and the light beam reflected by the reflecting surface 25 is again converged by a convergent lens 23 and a second cylindrical lens 22 to thereby form point images on a scanning surface 26. This method is different from that disclosed in the above-mentioned Japanese Patent Publication No. 52-28666 in that the second cylindrical lens 22 thereof is disposed behind the convergent lens 23. However, both methods are essentially the same in theory in that although the direction of the light beam reflected by the reflecting surface is varied due to the aforementioned parallelism errors, the light beam is focussed on a constant position on the scanning surface 26 with loci of the scan lines coincident with each other at all times.
Since the second cylindrical lens 22 is disposed between the convergent lens 23 and the scanning surface 26, the following inequality must be established: EQU f.sub.22 &lt;f.sub.23. (2a)
Moreover, in order to form a locus of a point images focussed on the scanning surface 26 along a circle, the following relationship must be established: EQU f.sub.21 .multidot.f.sub.22 =f.sub.23.sup.2, (2b)
where f.sub.21 and f.sub.22 are the focal lengths of the first and second cylindrical lenses 21 and 22, respectively, and f.sub.23 is the focal length of the convergent lens 23. For example, when f.sub.23 =50 mm, the focal length f.sub.21 must be about 83 to 71 mm. That is, the focal length f.sub.21 of the first cylindrical lens can be rather long to thereby increase the depth of focus. As a result, the variation of the convergence of the light beam on the scanning surface 26 due to displacement of the reflection position of the light beam generated together with the rotation of the reflecting surface can be reduced. However, since the second cylindrical lens must still be a short focal length lens, the above-noted difficulties in manufacturing such a lens of high quality remain.